Pressure vessels are commonly used for containing a variety of fluids under pressure, such as storing oxygen, natural gas, nitrogen, hydrogen, propane and other fuels, for example. Suitable container materials include laminated layers of wound fiberglass filaments or other synthetic filaments bonded together by a thermosetting or thermoplastic resin. A polymeric or other non-metal resilient liner or bladder often is disposed within the composite shell to seal the vessel and prevent internal fluids from contacting the composite material. The composite construction of the vessels provides numerous advantages such as lightness in weight and resistance to corrosion, fatigue and catastrophic failure. These attributes are due to the high specific strengths of the reinforcing fibers or filaments that are typically oriented in the direction of the principal forces in the construction of the pressure vessels.
FIGS. 1 and 2 illustrate an elongated pressure vessel 10, such as that disclosed in U.S. Pat. No. 5,476,189, which is hereby incorporated by reference. Vessel 10 has a main body section 12 with end sections 14. A metal boss 16 (e.g., aluminum) is provided at one or both ends of the vessel 10 to provide a port for communicating with the interior of the vessel 10. The vessel 10 is formed from an inner polymer liner 20 covered by an outer composite shell 18. In this case, “composite” means a fiber reinforced resin matrix material, such as a filament wound or laminated structure. The composite shell 18 resolves all structural loads and the plastic liner 20 provides a gas barrier.
The liner 20 has a generally hemispheroidal end section 22 with an opening 24 aligned within an opening 26 in the outer composite shell 18. Boss 16 is positioned within the aligned openings and includes a neck portion 28 and a radially outwardly projecting flange portion 30. The boss 16 defines a port 32 through which fluid at high pressure may be communicated with the interior of the pressure vessel 10.
In some applications, vessel 10 is used as an accumulator. This application involves high cycle lives (pressurization/depressurizations) of vessel 10. The neck 28 and flange 30 of the aluminum boss 16 are rigid. However, under pressure, the composite shell 18 tends to strain. If composite shell 18 is in direct contact with the metal material of boss 16, such strain may result in spalling of the composite shell 18 during cycling of the accumulator due to wear on the composite shell 18 by the boss 16.